1. Field of the Invention
This invention relates generally to methods and apparatus for fluidized catalytic cracking (FCC) units. More specifically this invention relates to methods for separating catalyst from product vapors in an FCC reaction zone.
2. Description of the Prior Art
The fluidized catalytic cracking of hydrocarbons is the main stay process for the production of gasoline and light hydrocarbon products from heavy hydrocarbons such as vacuum gas oils. Large hydrocarbon molecules associated with the heavy hydrocarbon feed are cracked to break large hydrocarbon chains thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed.
The basic equipment or apparatus for the fluidized catalytic cracking of hydrocarbons has been in existence since the early 1940's. The basic component of the FCC process include a reactor, a regenerator and a catalyst stripper. The reactor includes a contact zone where the hydrocarbon feed is contacted with a particulate catalyst and a separation zone where product vapors from the cracking reaction are separated from the catalyst. Further product separation takes place in a catalyst stripper that receives catalyst from the separation zone and removes entrained hydrocarbons from the catalyst by countercurrent contact with steam or another stripping medium. The FCC process is carried out by contacting the starting material, whether it be vacuum gas oil, reduced crude or another source of relatively high boiling hydrocarbons with a catalyst made up of a finely divided or particulate solid material. The catalyst is transported like a fluid by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport. The contact of the oil with fluidized material catalyses the cracking reaction. During the cracking reaction coke is deposited on the catalyst.
Coke is comprised of hydrogen and carbon and can include other materials in trace quantities such as sulfur and metals that enter the process with the starting material. Coke interferes with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place. Catalyst is transferred from the stripper to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content, relative to the catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then again to the reaction zone. The fluidized catalyst, as well as providing a catalytic function, acts as a vehicle for the transfer of heat from zone to zone. Catalyst exiting the reaction zone is spoken of as being spent, i.e., partially deactivated by the deposition of coke upon the catalyst. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
The rate of conversion of the feedstock within the reaction zone is controlled by regulation of the temperature of the catalyst, activity of the catalyst, quantity of the catalyst (i.e., catalyst to oil ratio) and contact time between the catalyst and feedstock. The most common method of regulating the reaction temperature is by regulating the rate of circulation of catalyst from the regeneration zone to the reaction zone which simultaneously produces a variation in the catalyst to oil ratio as the reaction temperatures change. That is, if it is desired to increase the conversion rate an increase in the rate of flow of circulating fluid catalyst from the regenerator to the reactor is effected. Since the catalyst temperature in the regeneration zone is usually held at a relatively constant temperature, significantly higher than the reaction zone temperature, any increase in catalyst flux from the relatively hot regeneration zone to the reaction zone affects an increase in the reaction zone temperature.
The hydrocarbon product of the FCC reaction is recovered in vapor form and transferred to product recovery facilities. These facilities normally comprise a main column for cooling the hydrocarbon vapor from the reactor and recovering a series of heavy cracked products which usually include bottom materials, cycle oil, and heavy gasoline. Lighter materials from the main column enter a concentration section for further separation into additional product streams.
The catalyst particles employed in an FCC process have a large surface area, which is due to a great multitude of pores located in the particles. As a result, the catalytic materials retain hydrocarbons within their pores and upon the external surface of the catalyst. Although the quantity of hydrocarbon retained on each individual catalyst particle is very small, the large amount of catalyst and the high catalyst circulation rate which is typically used in a modem FCC process results in a significant quantity of hydrocarbons being withdrawn from the reaction zone with the catalyst.
Therefore, it is common practice to remove, or strip, hydrocarbons from spent catalyst prior to passing it into the regeneration zone. It is important to remove retained spent hydrocarbons from the spent catalyst for process and economic reasons. First, hydrocarbons that entered the regenerator increase its carbon-burning load and can result in excessive regenerator temperatures. Stripping hydrocarbons from the catalyst also allows recovery of the hydrocarbons as products. Avoiding the unnecessary burning of hydrocarbons is especially important during the processing of heavy (relatively high molecular weight) feedstocks, since processing these feedstocks increases the deposition of coke on the catalyst during the reaction (in comparison to the coking rate with light feedstocks) and raises the combustion load in the regeneration zone. Higher combustion loads lead to higher temperatures which at some point may damage the catalyst or exceed the metallurgical design limits of the regeneration apparatus.
The most common method of stripping the catalyst passes a stripping gas, usually steam, through a flowing stream of catalyst, countercurrent to its direction of flow. Such steam stripping operations, with varying degrees of efficiency, remove the hydrocarbon vapors which are entrained with the catalyst and hydrocarbons which are adsorbed on the catalyst. Contact of the catalyst with a stripping medium may be accomplished in a simple open vessel as demonstrated by U.S. Pat. No. 4,481,103.
The efficiency of catalyst stripping is increased by using vertically spaced baffles to cascade the catalyst from side to side as it moves down a stripping apparatus and countercurrently contacts a stripping medium. Moving the catalyst horizontally increases contact between the catalyst and the stripping medium so that more hydrocarbons are removed from the catalyst. In these arrangements, the catalyst is given a labyrinthine path through a series of baffles located at different levels. Catalyst and gas contact is increased by this arrangement that leaves no open vertical path of significant cross-section through the stripping apparatus. Further examples of these stripping devices for FCC units are shown in U.S. Pat. Nos. 2,440,620; 2,612,438; 3,894,932; 4,414,100; and 4,364,905. These references show the typical stripper arrangement having a stripper vessel, a series of baffles in the form of frusto-conical sections that direct the catalyst inwardly onto a baffle in a series of centrally located conical or frusto conical baffles that divert the catalyst outwardly onto the outer baffles. The stripping medium enters from below the lower baffle in the series and continues rising upward from the bottom of one baffle to the bottom of the next succeeding baffle. Variations in the baffles include the addition of skirts about the trailing edge of the baffle as depicted in U.S. Pat. No. 2,994,659 and the use of multiple linear baffle sections at different baffle levels as demonstrated by FIG. 3 of U.S. Pat. No. 4,500,423. A variation in introducing the stripping medium is shown in U.S. Pat. No. 2,541,801 where a quantity of fluidizing gas is admitted at a number of discrete locations.
As the development of FCC units has advanced, temperatures within the reaction zone were gradually raised. It is now commonplace to employ temperatures of about 525.degree. C. At higher temperatures, there is generally a loss of gasoline components as these materials crack to lighter components by both catalytic and strictly thermal mechanisms. At 525.degree. C., it is typical to have 1% of the potential gasoline components thermally cracked into lighter hydrocarbon gases. As temperatures increase, to say 550.degree. C., most feedstocks can lose up to 6% or more of the gasoline components to thermal cracking. However, the loss of gasoline can be offset by the often more desirable production of light olefins.
One improvement to FCC units, that has reduced the product loss by thermal cracking, is the use of riser cracking. In riser cracking, regenerated catalyst and starting materials enter a pipe reactor and are transported upward by the expansion of the gases that result from the vaporization of the hydrocarbons, and other fluidizing mediums if present upon contact with the hot catalyst. Riser cracking provides good initial catalyst and oil contact and also allows the time of contact between the catalyst and oil to be more closely controlled by eliminating turbulence and backmixing that can vary the catalyst residence time. An average riser cracking zone today will have a catalyst to oil contact time of 1 to 5 seconds. A number of riser reaction zones use a lift gas as a further means of providing a uniform catalyst flow. Lift gas is used to accelerate catalyst in a first section of the riser before introduction of the feed and thereby reduces the turbulence which can vary the contact time between the catalyst and hydrocarbons.
In most reactor arrangements, catalysts and conversion products still enter a large chamber for the purpose of initially disengaging catalyst and hydrocarbons. The large open volume of the disengaging vessel exposes the hydrocarbon vapors to turbulence and backmixing that continues catalyst contact for varied amounts of time and keeps the hydrocarbon vapors at elevated temperatures for a variable and extended amount of time. Thus, thermal cracking can be a problem in the disengaging vessel. A final separation of the hydrocarbon vapors from the catalyst is performed by cyclone separators that use centripedal acceleration to disengage the heavier catalyst particles from the lighter vapors which are removed from the reaction zone.
In order to minimize thermal cracking in the disengaging vessel, a variety of systems for directly connecting the outlet of the riser reactor to the inlet of a cyclone are suggested in the prior art. A majority of the hydrocarbon vapors that contact the catalyst in the reaction zone are separated from the solid particles by ballistic and/or centrifugal separation methods within the reaction zone. Directly connecting the inlet of a first cyclone and the outlet the first cyclone to the inlet of a second cyclone in what has been termed a "direct connected cyclone system" can greatly reduce thermal cracking of hydrocarbons. Unfortunately in most cases direct connected cyclones will increase the complexity of operating an FCC unit. When the cyclones are directly connected to the riser any pressure surges that normally occur in the FCC unit can cause the cyclones to malfunction and lead to the carry-over of catalyst into the main column and separation facilities for the recovery of the product. A number of different riser and cyclone arrangements are shown in the prior art to increase the reliability of the cyclone operation when the riser is directly connected thereto.
One way in which to overcome the problem of pressure surges and catalyst carry over is to connect a separation device having a large capacity to the outlet of the riser. Such a separation device is shown in FIG. 8 of U.S. Pat. No. 4,689,206. This separation device provides a disengagement of the catalyst and product vapor mixture before the mixture enters the relatively small volume of an ordinary cyclone. Due to its large volume the separation device is not easily overloaded and ordinary pressure surges will not interrupt its operation. However such large separation devices suffer from low separation efficiencies that increase the particle load on the downstream cyclones or require the use of two stage cyclones or must have a relatively long length to provide a high separation efficiency. Reduced efficiencies are in large part caused by the reentrainment of catalyst particles with the gas as it flows out of the separation device.